1. Field of the Invention
This invention relates to a major aluminum tolerance gene, SbMATE (for Sorghum bicolor member of the multidrug and toxic compound extrusion transporter family), that is responsible for the AltSB locus (for aluminum tolerance in Sorghum bicolor), cloned from sorghum along with its native promoter and regulatory regions, a construct containing the gene and a constitutive promoter, a vector containing the construct, and a method of transforming a plant utilizing the construct and vector, and plants, including staple crop plants, transformed with the gene construct having increased tolerance to aluminum toxicity.
2. Description of the Relevant Art
The tropics and subtropics are extremely important food producing regions, particularly for many developing countries. However, agriculture on the acid soils that are prevalent in these areas is seriously challenged by limitations to plant yield caused by drought, mineral nutrient deficiencies such as phosphorus deficiency, and in particular, aluminum toxicity. Aluminum (Al) is ubiquitous in soils and, at pH values below 5.0, is solubilized into the soil solution as the highly phytotoxic Al3+ species, which inhibits root growth and damages root systems (Kochian, L. V. 1995. Annu. Rev. Plant Biol. 46: 237-260). Hence, aluminum toxicity is a primary limitation for crop production in many developing countries, including 38% of the farmland in Southeast Asia, 31% in Latin America and 20% of the arable lands in East Asia and Sub-Saharan Africa (Wood et al. 2000. In Pilot Analysis of Global Ecosystems: Agroecosystems, International Food Policy Research Institute and the World Resources Institute, Washington, D.C.), thus reducing food security in parts of the world where it is most tenuous.
A major aluminum tolerance mechanism has been identified in plants based on aluminum-activated organic acid release from the root apex, which is the site of aluminum phytotoxicity (Ryan et al. 1993. J. Exp. Bot. 44: 437-446). Depending on the plant species, the organic acids malate, citrate, or oxalate are released from the roots in response to aluminum exposure and form stable, nontoxic complexes with Al3+ cations (Ma et al. 2001. Trends Plant Sci. 6: 273-278). A considerable body of physiological evidence in support of this mechanism exists in the literature (see Kochian et al. 2004. Annu. Rev. Plant Biol. 55: 459-493 and references therein) and it is now generally accepted that with regards to this Al tolerance mechanism, aluminum activates a plasma membrane organic acid transporter, and that this transporter plays a central role in aluminum tolerance (Delhaize and Ryan. 1995. Plant Physiol. 107: 315-321; Ryan et al. 2001. Annu. Rev. Plant Physiol. Plant MoL Biol. 52: 527-560; Kochian et al. 2004, supra).
Recently, the first aluminum (Al) tolerance gene, ALMT1, was isolated and shown to encode an Al activated malate transporter (Sasaki et al. 2004. Plant J. 37: 645-653). ALMT1 was found to be a member of a novel family of membrane proteins, and based on genetic mapping of the ALMT1 gene, it was shown to most likely correspond to AltBH, a major Al tolerance locus in wheat and other members of the Tritceae tribe (Sasaki et al., supra; Raman et al. 2005. Genome 48: 781-791). Subsequently, a homolog of ALMT1 was shown to also confer Al tolerance via Al-activated root malate exudation in Arabidopsis (Hoekenga et al. 2006. Proc. Natl. Acad. Sci. USA 103: 9738-9743).
A second constraint on acid soils is phosphorous (P) deficiency, which is caused by P fixation with Al and Fe oxides on the surface of clay minerals in acid soils. Hence P availability is a second major factor limiting crop production on acid soils (Sanchez et al. 1997. In: Replenishing Soil Fertility in Africa, ed. R Buresh, P Sanchez, F Calhoun, pp. 1-46). Because of the low availability of this essential mineral nutrient, P, plants have evolved a number of adaptive mechanisms to acquire P from the soil. One major such adaptive mechanism is the release of organic acids, primarily citrate and malate, from roots (Neumann G. and Martinoia, E. 2002. Trends Plant Sci. 7: 162-167). These released organic acids can desorb P from mineral surfaces, solubilizing it from associations with Al, Fe and Ca oxides and hydroxides via metal complexation. Hence, a transporter such as SbMATE that can facilitate the efflux of citric acid from roots could significantly increase the ability of crop plants to acquire P from acid soils with low P availability.
There is a real need in both developing and developed countries to better understand Al tolerance mechanisms and associated genes and their effect on crop plants. Genes that confer enhanced, Al-activated organic acid (citrate, malate, or oxalate) release to crop plants need to be identified and evaluated for their effect in enabling increased Al tolerance in a wide range of crop species grown in acid soils worldwide. The utilization of genes which contribute to a plant's tolerance to the Al present in acid soils results in transgenic plants with a mechanism for achieving high yield in acid soils where soluble Al3+ are found. Thus, such genes and constructs can ensure yield stability for plants grown on acid soils, i.e., Al-tolerant plants make possible the utilization of marginal lands for agriculturally and commercially important staple crop production.